pred

A prediction market for you and your friends. Full-stack TypeScript: an Express + SQLite backend with a real order book, and a responsive React frontend that works on desktop and mobile.

⚠️ Just for fun — not real money. This is a hobby project for playing prediction markets with friends. Every account is funded with play money only. There is no real currency, no deposits, no withdrawals, and nothing of monetary value anywhere in the app. The "$" amounts, balances, payouts, and leaderboard are all imaginary points. This is not a gambling service, a financial product, or investment advice, and it is not affiliated with any real exchange or prediction market. Use it for entertainment among friends only.

How it works

• Everyone signs up and gets $1,000 of play money — fake points with no real-world value whatsoever. The first account created becomes the admin.
• Anyone can create a market — a yes/no question like "Will Sam finish the marathon?"
• Markets trade binary shares priced in cents (1–99¢). A YES share pays $1.00 if the market resolves YES; a NO share pays $1.00 if it resolves NO.
• Trading happens on a standard limit order book. Every order is a buy of one side: a YES buy at 60¢ crosses with a NO buy at 40¢ (or better), and the pair of buyers mint a YES/NO share pair whose $1.00 is escrowed until resolution. Orders execute at the resting order's price, so takers keep any price improvement.
• To exit a position, buy the opposite side — whenever you hold both YES and NO shares, matched pairs are automatically redeemed for $1.00 each.
• Only admins can resolve markets: YES, NO, or VOID (refunds 50¢ per share to each side). Resolving cancels and refunds all open orders.
• A leaderboard ranks everyone by net worth (cash + open orders + positions marked at the last traded price).
• A portfolio page shows all of your open orders (cancellable from there) and every position you hold across markets.

Development

Requires Node 22+.

npm install
npm run dev

This runs the API server on http://localhost:3000 and the Vite dev server on http://localhost:5173 (with /api proxied to the backend). Open the Vite URL.

Production / deployment

npm install
npm run build
npm start          # serves the built client + API on PORT (default 3000)

Or with Docker:

docker build -t pred .
docker run -p 3000:3000 -v pred-data:/data pred

Configuration (environment variables):

Variable	Default	Purpose
PORT	3000	HTTP port
DATABASE_PATH	data/pred.db	SQLite database file
JWT_SECRET	auto-generated	Session signing key (persisted in DB if unset)
SIGNUP_CODE	unset	If set, registration requires this invite code

The whole app is a single Node process with a SQLite file, so it deploys anywhere you can run a container or a Node server (Fly.io, Railway, Render, a $5 VPS, ...). The only state is the SQLite file, so point DATABASE_PATH at storage that survives restarts.

Deploying to Fly.io

Fly machines have ephemeral filesystems, so the database goes on a persistent Fly volume. A ready-to-use fly.toml is included — it mounts a volume at /data and sets DATABASE_PATH=/data/pred.db:

fly launch --no-deploy        # creates the app; keep the provided fly.toml
fly volumes create pred_data --size 1
fly secrets set SIGNUP_CODE=something-only-your-friends-know
fly deploy

Register your own account immediately after the first deploy — the first account created becomes the admin. Setting SIGNUP_CODE keeps strangers who stumble on the URL from joining; share the code with your friends along with the link.

Notes:

• Run a single machine (the default with one volume): SQLite doesn't replicate across machines. fly scale count 1 if you ever scaled up.
• The volume persists across deploys and restarts; everything else in the container is rebuilt each deploy.
• Optionally fly secrets set JWT_SECRET=... — otherwise a generated secret is persisted inside the database and sessions survive restarts anyway.

Admin notes

The first registered user is automatically the admin, and only admins can resolve markets. There are two ways to make additional users admins:

Option 1: via the API (works anywhere)

Any existing admin can promote another user. First log in as the admin to get a token, find the target user's id on the leaderboard, then call the promote endpoint:

APP=https://your-app.fly.dev        # or http://localhost:3000

# 1. Log in as an admin and grab the token
TOKEN=$(curl -s -X POST $APP/api/auth/login \
  -H 'Content-Type: application/json' \
  -d '{"username":"YOUR_ADMIN_USERNAME","password":"YOUR_PASSWORD"}' \
  | python3 -c "import sys,json;print(json.load(sys.stdin)['token'])")

# 2. Find the user's id (the leaderboard lists everyone with their id)
curl -s $APP/api/leaderboard -H "Authorization: Bearer $TOKEN" | python3 -m json.tool

# 3. Promote user id 4 to admin
curl -s -X POST $APP/api/admin/users/4/promote -H "Authorization: Bearer $TOKEN"

Option 2: directly in the database

Set the is_admin flag on the users table. Locally (with the server stopped, or it's fine while running — SQLite handles it):

sqlite3 data/pred.db "UPDATE users SET is_admin = 1 WHERE username = 'friend';"

On Fly.io, the database lives on the volume at /data/pred.db. The container doesn't ship the sqlite3 CLI, but the server's own driver works:

fly ssh console
cd /app
node -e "require('better-sqlite3')('/data/pred.db')
  .prepare('UPDATE users SET is_admin = 1 WHERE username = ?').run('friend')"

To demote an admin, use the same database update with is_admin = 0 (there is no demote API endpoint).

Resetting a forgotten password

An admin can set a new password for any user (find the user's id on the leaderboard, as above):

curl -s -X POST $APP/api/admin/users/4/reset-password \
  -H "Authorization: Bearer $TOKEN" \
  -H 'Content-Type: application/json' \
  -d '{"password":"new-temporary-password"}'

Tell the user to log in with the temporary password. (There is no self-service change-password flow yet, so pick something they're happy to keep or reset it again later.)

Disclaimer

This project is provided for fun and educational purposes only. It is a toy prediction market played with play money — points that have no monetary value and cannot be bought, sold, redeemed, deposited, or withdrawn. Nothing in this app involves real currency or anything of value.

• It is not a gambling, betting, or wagering service.
• It is not a financial product, brokerage, exchange, or security, and nothing here is financial, investment, or trading advice.
• It is not affiliated with, endorsed by, or connected to any real prediction market, exchange, or financial institution.
• Any resemblance of the "$", balances, prices, or payouts to real money is purely cosmetic.

The software is provided "as is", without warranty of any kind. You are responsible for ensuring that running it among your friends complies with the laws and regulations that apply to you. Use it purely for entertainment.